Дисертації з теми "Plasma sprayed ceramics"

Orientadores: Cecília Amélia de Carvalho Zavaglia, Carmo Roberto Pelliciari de Lima<br>Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica<br>Made available in DSpace on 2018-08-20T04:15:56Z (GMT). No. of bitstreams: 1 Goncalves_FernandodeAlmeida_D.pdf: 4153645 bytes, checksum: fe7e541e43fe3f147ac65eff5bc547f0 (MD5) Previous issue date: 2012<br>Resumo: A necessidade de melhorar as características superficiais da liga Ti-4Al-6V usada em implantes ortopédicos, levou à pesquisas no sentido de estudar a modificação da superfície dos implantes através da deposição de revestimentos cerâmicos resistentes à corrosão, ao desgaste e biocompatíveis, por vários métodos: eletroquímica, física a vapor, eletroforética, por sol-gel, biomimética e por aspersão térmica a plasma, entre outras. A aspersão térmica a plasma é o processo mais utilizado comercialmente, pois é rápido e reprodutível. As biocerâmicas mais utilizadas para revestimento, com sucesso, são as de fosfato de cálcio (hidroxiapatita-HA, betafosfato tricálcico [o -TCP] e uma mistura dessas duas fases). A adesão dessas biocerâmicas ao tecido ósseo possuí bom desempenho é bastante discutida na literatura. A comunidade médica tem algumas restrições ao uso desses implantes metálicos revestidos por cerâmicas, com relação à interface metal-cerâmica, cuja adesão é considerada baixa. Neste trabalho foi estudado a adesão metal/cerâmica em implantes revestidos por aspersão térmica à plasma (ATP) com cerâmica, mais especificamente liga de titânio (Ti-6Al-4V) revestida com titânia (TiO2) sem e com tratamento térmico a vácuo, com o intuito de verificar uma possível melhoria nessa adesão. A cerâmica utilizada neste trabalho foi a titânia, uma vez que é uma cerâmica biocompatível e osseointegrável, que é bastante utilizada para revestir implantes. Para a execução do trabalho foram confeccionados diversos corpos de prova, os quais foram revestidos por titânia por aspersão térmica à plasma, tratados termicamente e caracterizados segundo procedimentos contidos em normas e trabalhos científicos correlacionados que norteiam o assunto proposto. Foram utilizadas as seguintes técnicas de caracterização: análise granulométrica dos pós cerâmicos, difração de raios X, verificação da porosidade, microscopia óptica e eletrônica de varredura, verificação da rugosidade das amostras. Especial ênfase foi dada aos ensaios de adesão metal-cerâmica: ensaios de adesão por tração, flexão e riscamento. Tentou-se utilizar, sem sucesso, os ensaios de microdureza para avaliar a adesão. Como resultados, verificou-se uma boa qualidade nas amostras da liga Ti-4Al-6V revestidas por titânia por aspersão térmica a plasma. Através das três técnicas de avaliação da adesão, verificou-se uma ligeira melhoria dessas características pelo tratamento térmico a vácuo, no ensaio de riscamento a carga crítica aumentou de 53N para 62N, no ensaio de tração a tensão de fratura aumentou de 10,5MPa para 17,4MPa e no ensaio de flexão a tensão de fratura aumentou de 153,3MPa para 193,1MPa. Comparando com valores encontrados na literatura, a adesão Ti-4Al-6V/TiO2 ficou superior a Ti-4Al-6V/HA nas mesmas condições de deposição<br>Abstract: The need to improve the surface characteristics of the alloy Ti-4Al-6V used in orthopedic implants, increase researches with the focus into the surface modification, using deposition of ceramic coatings resistant to corrosion, wear and more biocompatible by various methods: electrochemistry, physical vapor, electrophoretic, by sol-gel, biomimetics and by thermal plasma, among others. The plasma spray technique is the process more used commercially because it is fast and reproducible. The bioceramics most used for coating, are calcium phosphate (hydroxyapatite-HA, beta-tricalcium phosphate, o -TCP and a mixture of these two phases). The adhesion of these bioceramics to the bone tissue is quite discussed in literature, presenting a good performance. However, the medical community has some restrictions on the use of metal implants with ceramic coating, since the metal-ceramic interface is considered low. In this work the aim is to study the adhesion metal/ceramic-coated implants in the thermal spray plasma (ATP), using titanium alloy (Ti-6Al-4V) coated with titania (TiO2) with and without vacuum heat treatment in order to check a possible improvement this adherence. The ceramic used in this work was the titania, since it is biocompatible and bioactive, and it is quite used to coat dental implants. For the execution of the work were made several specimens, which were coated with titania by plasma thermal spray, heat treated and characterized according to procedures and standards contained in scientific papers related to guide the proposed subject. The following characterization techniques it were used: particle size analysis of ceramic powders, X-ray diffraction, analysis of the porosity, optical microscopy and scanning electron microscopy, scanning the roughness of the samples. Special emphasis was given to tests of metal-ceramic adhesion: adhesion assays for tensile, bending and scratching. Micro hardness tests carried out, however the results were not significant. The summary of this project were that this alloy present a good quality coated by titania. The methods of increase the adhesion, showed a slight improvement of these characteristics by vacuum heat treatment, the result of the scratching test showed that the critical load increase to 62N instead 53N, when analyzed the results of tensile test it also had an increase 17,4MPa instead 10,5 MPa, the bending test presented higher results 193,1MPa in contrast to 153,3MPa for the samples without treatment. Compared with values found in the literature, the adherence of Ti-4Al-6V/TiO2 was greater than Ti-4Al-6V/HA<br>Doutorado<br>Materiais e Processos de Fabricação<br>Doutor em Engenharia Mecânica

La projection plasma de suspensions (SPS) est un procédé de revêtement de surface qui consiste à injecter une suspension (particules solides d’environ 1 μm ou moins, dispersées dans une phase liquide) dans un jet de plasma énergétique. Les particules sont chauffées, accélérées en direction d’un substrat, écrasées et soumises à une solidification très rapide (de l’ordre de 106 K.s-1). Couche après couche, un dépôt se forme en surface du substrat et lui apporte de nouvelles propriétés fonctionnelles. Cette variante de la projection plasma conventionnelle permet la fabrication de revêtements avec des épaisseurs plus fines de quelques dizaines de μm et une échelle microstructurale réduite, pouvant conduire à améliorer, par exemple, les performances de dureté ou de conductivité thermique des dépôts. Bien que ce procédé soit étudié depuis le milieu des années 1990 et connaisse un intérêt grandissant, les applications industrielles ne sont pas finalisées et leur développement nécessite d’être poursuivi. En effet, l’injection d’une suspension dans un jet thermique conduit à des phénomènes complexes tels que la fragmentation des gouttes de suspension ou encore l’évaporation de la phase liquide. A ce jour, ces mécanismes ne sont pas parfaitement compris et maîtrisés et méritent d’être étudiés pour comprendre les interactions de ces fines particules avec le plasma. Les travaux décrits dans ce mémoire s’intéressent au cas de la projection SPS de céramiques avec un atomiseur bi-fluide comme système d’injection. Deux matériaux ont été choisis : l’alumine, connue pour sa difficulté à être projetée conventionnellement et dont la formation de phases cristallines particulières constitue une source d’informations sur l’histoire thermique des particules, ainsi que l’yttrine, qui permet de confirmer les tendances observées pour l’alumine. Dans un premier temps, l’optimisation de l’injection de la suspension a été effectuée en travaillant sur deux axes. Le premier axe concerne la formulation des suspensions, qui a conduit à l’obtention, avec différentes phases liquides, de suspensions stables et dispersées, de propriétés parfaitement connues. De telles suspensions assurent une reproductibilité du procédé à ce niveau et limitent le bouchage du système d’injection. Le deuxième axe porte sur la conception mécanique en trois étapes d’un atomiseur pneumatique approprié au procédé SPS. Cette étude a commencé par la caractérisation d’une buse commerciale notamment par des tests d’injection de suspension dans le plasma. Les tests étant peu concluants, l’étude s’est poursuivie par la mise au point d’une nouvelle géométrie d’atomiseur inspirée du modèle commercial. Les essais ont conduit à la réalisation de cordons et de dépôts satisfaisants. Cette étude s’est terminée enfin par l’optimisation de sa géométrie grâce à la mise en évidence de l’influence de plusieurs paramètres-clé sur les caractéristiques du jet atomisé. Dans un second temps, des outils de diagnostic ont été mis en oeuvre pour mesurer la qualité de l’injection. Le jet de suspension a été caractérisé en termes de géométrie et de tailles de gouttes, respectivement par ombroscopie et diffraction laser. L’ombroscopie a été réutilisée pour l’optimisation de l’injection de la suspension dans le plasma en permettant le réglage en temps réel des pressions d’entrée de l’atomiseur. Les propriétés des particules en vol ont ensuite été étudiées grâce à des collectes de particules sur substrat et à la vélocimétrie par images de particules (PIV). Cet outil a apporté des informations complémentaires sur l’injection de la suspension. Enfin, les revêtements obtenus ont été caractérisés en termes de morphologie (MEB), taux de porosité (analyse d’images MEB et USAXS) et de phases cristallines (DRX et EBSD). Le couplage des informations obtenues entre ces différentes techniques a permis de faire ressortir le rôle de la phase liquide et de la charge massique sur la microstructure<br>Suspension plasma spray (SPS) is a surface coating process that consists in injecting a suspension (solid particles of about 1 μm or less, dispersed in a liquid phase) in a high-energy plasma flow. Particles are heated, accelerated towards a substrate, flattened and submitted to a rapid solidification (order of 106 K.s-1). Layer after layer, a coating is formed on the substrate surface and brings new functional properties. This variation of the conventional plasma spray process allows the manufacturing of coatings with finer thickness of few tens of μm and a reduced structural scale that can lead to improved coating properties, like hardness or thermal conductivity. Even though this process has been studied since the middle of the 1990’S and known a fast-growing interest, industrial applications are not finalized and their development needs to be pursued. Indeed, the suspension injection in a thermal jet leads to complex phenomena such as suspension droplet fragmentation or liquid phase evaporation. Up to now, these mechanisms are not perfectly understood and controlled and deserve to be further studied to understand interactions between these fine particles and the plasma. This thesis focuses on the SPS process with ceramic suspensions and a twin-fluid nozzle as injection system. Two materials were chosen: alumina, known for its difficulty to be conventionally sprayed and whose crystalline phase formation represents a source of information about particle thermal history, and also yttria, in order to confirm the tendencies observed for alumina. Firstly, the suspension injection was optimized by working on two areas. The first area concerns suspension formulation. This led to obtain, with different liquid phases, stable and dispersed suspensions, whose properties are perfectly known. Such suspensions ensure reproducibility of the process at this level and limit the risk of injection system clogging. The second area is about the three-step mechanical conception of a pneumatic atomizer, adapted to the SPS process. This study began with the characterization of a commercial nozzle, in particular by testing the suspension injection into a plasma flow. Tests being little convincing, the study was carried on with the development of a new atomizer geometry, inspired from the commercial model. Trials drove to the manufacturing of satisfying spray beads and coatings. This study was finally completed with the optimization of this new geometry by highlighting the influence of several key parameters on the atomized jet features. Secondly, diagnostic tools were implemented to qualify the injection. Suspension jet was characterized in terms of geometry and droplet sizes, using respectively shadowgraphy and laser diffraction. Shadowgraphy was used again for optimizing the suspension injection into plasma by allowing the adjustment in real time of inlet atomizer pressures. In-flight particle properties were then studied thanks to particle collection onto a substrate and particle image velocimetry (PIV). This tool also provided additional information on the suspension injection. Finally, the resulting coatings were characterized in terms of morphology (SEM), porosity rate (SEM image analysis and USAXS) and crystalline phases (DRX and EBSD). The cross-checking of the information obtained with all these techniques brought out the role of the suspension liquid phase and of the mass load on the coating microstructure. These works contributed to enhance the knowledge about the SPS process and justified the use of a twin-fluid nozzle to obtain specific microstructures of coatings, whose functional characterizations have still to be done

Les céramiques sont sensibles à la fissuration lente qui résulte de l'effet conjoint entre un chargement mécanique et l'environnement (taux d'humidité et température). A partir d'études atomistiques disponibles dans la littérature, un modèle cohésif représentant localement la rupture assistée par l'environnement est proposé dans le cadre d'une formulation thermiquement activée. Nous montrons que cette description est capable de rendre compte de la fissuration lente en fatigue statique de monocristaux de céramiques, ainsi que la fissuration lente intergranulaire de polycristaux. Nous soulignons qu'une représentation de la fissuration lente avec la vitesse de propagation V en fonction du taux de restitutions d'énergie G rend compte des caractéristiques intrinsèques de la cinétique de rupture et est préférable à une présentation V-K. Le modèle cohésif permettant d'incorporer une longueur caractéristique dans la description, des effets de taille de grains sont explorés. La prise en compte des contraintes initiales d'origine thermique liées à l'élaboration est nécessaire pour prédire de manière réaliste l'accroissement du seuil de chargement en-dessous duquel aucune propagation n'a lieu ainsi que la résistance à la fissuration lente avec la taille de grains augmentant. La vitesse fissuration lente et le seuil de chargement K0 sont sensibles à l'environnement et notamment à la température et à la concentration d'eau. En augmentant la concentration d'eau et/ou la température, le seuil K0 diminue et la vitesse de fissuration lente augmente. Pour rendre compte de l'influence du taux d'humidité sur la fissuration lente, il est nécessaire de considérer une énergie d'activation ainsi qu'un seuil d'amorçage du mécanisme de réaction-rupture diminuant avec la concentration locale en eau. L'effet de la température est prédit de manière réaliste avec le modèle cohésif proposé et en tenant compte des contraintes initiales thermiques. Nous avons comparé les réponses en fissuration lente de l'alumine et de la zircone et montré qu'intrinsèquement et en l'absence de transformation de phase, la zircone résiste mieux à la fissuration lente que l'alumine. A partir de ces résultats, nous avons abordé l'étude de la fissuration lente de céramiques élaborées par projection plasma. Un endommagement initial de la microstructure à l'échelle des splats est observé sans qu'il n'influence la fissuration lente intra-splats en termes V-G<br>Ceramic materials are prone to slow crack growth (SCG)due to the combined effect of the mechanical loading and the environment (moisture and temperature).Based on atomistic studies available in the literature,a thermally activated cohesive model is proposed to represent the reaction-rupture mechanism underlying slow crack growth. The description is shown able to capture SCG under static fatigue on ceramic single crystals as well as intergranular SCG in polycrystals.We emphasize that the representation of SCG with the crack velocity versus the energy release rate G accounts for the intrinsic characteristics of SCG, which is preferable than a usual plot with V-K curves.The cohesive model incorporates a characteristic length scale, so that size effects can be investigated. SCG is grain size dependent with the decrease of the crack velocity at a given load level and improvement of the load threshold with the grain size. To capture this observation, account for the initial thermal stresses related to the processing is mandatory. SCG is also dependent on the concentration of water with an increase of the crack velocity and a decrease of the load threshold with the relative humidity increasing. To predict this effect, the cohesive description needs to account for activation energy and a threshold to trigger the reaction-rupture that depends on the concentration of water. The influence of the temperature on SCG shows an increase in the crack velocity and a decrease of the load threshold for SCG due to the reduction in the initial thermal stresses. The SCG behavior of the alumina and zirconia is compared. Zirconia exhibits a better resistance to SCG compared to that of alumina, in the absence of any phase transformation due to lower kinetics of its reaction-rupture. Based on these results, SCG is investigated in plasma sprayed ceramic. An initial damage at the scale of the splats is observed without effect on load threshold G0 for SCG in V-G plots

The dissertation is split into two parts. The first part will be focused on changes in material properties found at the nanoscale, as miscibility and electronic structure can change significantly with size. The formation of classically-immiscible bimetallic nanoparticles (BNPs) becomes favorable at the nanoscale and novel catalytic properties can emerge from the bimetallic alloying. The formation of alloyed and non-alloyed BNPs is achieved through pulse laser ablation (PLA) and a significant increase in catalytic activity is observed for both. Recently discovered, the increased activity in the non-alloyed BNPs, deemed multicomponent photocatalysis, is examined and the proposed mechanism discussed. The second part of the talk will focus on thermal barrier coatings (TBCs), which are advanced, multi-layered coatings used to protect materials in high temperature environments. MCrAlY (M=Ni, Co) bond coats deposited via atmospheric plasma spray (APS) are intrinsically rough and initially the roughness provides a high surface area platform for the mechanical interlocking of the yttria stabilized zirconia (YSZ) top coat, which provides the bulk of the thermal insulation. After high temperature exposure, a protective oxide scale forms at the top coat/bond coat interface however the convex asperities of the bond coat can grow non-α-Al2O3 type oxides that can be detrimental for coating lifetime. A surface modification technique that removes the asperities while leaving intact the concavities is used to examine the role that roughness distribution has on 1100°C APS coating lifetime. Lastly, recent work validating a modelling strategy for evaluating 900°C TBC lifetimes, which can typically surpass 25 kh, is presented. Differences in coating-substrate interdiffusion behavior over 5-20 kh of 900°C exposure are discussed and reproduced with Thermo- Calc/DICTRA for three superalloys (1483, 247, X4) deposited with high velocity oxy fuel (HVOF) NiCoCrAlY coatings.

This thesis describes the design, manufacture and characterisation of thick vacuum plasma sprayed tungsten (W) coatings on steel substrates. Fusion is a potentially clean, sustainable, energy source in which nuclear energy is generated via the release of internal energy from nuclei. In order to fuse nuclei the Coulomb barrier must be breached - requiring extreme temperatures or pressures – akin to creating a ‘star in a box’. Tungsten is a promising candidate material for future fusion reactors due to a high sputtering threshold and melting temperature. However, the large coefficient of thermal expansion mismatch with reactor structural steels such as the low activation steel Eurofer’97 is a major manufacturing and in-service problem. A vacuum plasma spraying approach for the manufacture of tungsten and tungsten/steel graded coatings has been developed successfully. The use of graded coatings and highly textured 3D interface surfi-sculpt substrates has been investigated to allow the deposition of thick plasma sprayed tungsten coatings on steel substrates. Finite element models have been developed to understand the residual stresses that develop in W/steel systems and made use of experimental measurements of coating thermal history during manufacture and elastic moduli measured by nano-indentation. For both the graded and surfi-sculpt coating, the models have been used to understand the mechanism of residual stress redistribution and relief in comparison with simple W on steel coatings, particularly by consideration of stored strain energy. In the case of surfi-sculpt W coatings, the patterned substrate gave rise to regular stress concentrating features, and allowed 2mm thick W coatings to be produced reproducibly without delamination. Preliminary through thickness residual stress measurements were compared to model predictions and provided tentative evidence of significant W coating stress relief by regulated coating segmentation.

Yttria Partially Stabilized Zirconia (Y-PSZ) plasma-sprayed coatings are widely used in turbine engines as thermal barrier coatings. However, in diesel engines Y-PSZ TBCs have not met with wide success. To reach the desirable temperature of 850-900˚C in the combustion chamber from the current temperature of 400-600˚C, a coating with a thickness of approximately 1mm is required. This introduces different considerations than in the case of turbine blade coatings, which are on the order of 100µm thick. Of the many factors affecting the durability and failure mechanism of TBCs, in service and residual stresses play an especially important role as the thickness of the coating increases. For decreasing the residual stress in the system, a multi-layer coating is helpful. The design of a multilayer coating employing relatively low cost materials with complementary thermal properties is described. Numerical models were used to describe the residual stress after deposition and under operating conditions for a multilayer coating that exhibited the desired temperature gradient. Results showed that the multilayer coating had a lower maximum stress under service conditions than a conventional Y-PSZ coating. Model validation with experiments showed a good match between the two.

Thesis (M.S.)--University of Wisconsin--Madison, 1996.<br>Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 65-67).

High fuel utilization SOFCs could eliminate emissions from systems that include afterburners and potentially be suitable for carbon sequestration, while producing electricity more efficiently. Current fuel utilization operating points are typically chosen at approximately 85% for Ni-cermet anodes because higher fuel utilization frequently results in the formation of nickel oxide and reduces drastically the performance of the SOFC. In this work the feasibility of an in-plane graded anode architecture with a transition from a material with high catalytic activity to materials more stable under high fuel utilization conditions was evaluated through a steady-state SOFC finite element model. Thereafter, plasma spraying of solution precursor feedstock (SPPS) and suspension feedstock (SPS) was used to fabricate ceramic coatings that could potentially be used as SOFC anodes for high fuel utilization conditions. Microstructural, electrical and electrochemical properties of LST, LSBT and LSFCr coatings with additions of carbon black pore former were investigated.

碩士<br>中原大學<br>化學工程研究所<br>95<br>Air plasma spraying process was wildly used to produce ceramic membranes and thermal barrier coatings in industry field. It has been well-known that the particles melting status and its in-flight velocity was the most important factors that affect the lamellar formation and porosity. The interaction of particles with the plasma jet is crucial to the coating properties. However, direct measurement of particle heating history was difficult in experiments because of its residence time is extremely short. All the information concerning particle melting state can’t thoroughly get from on-line monitor facility. Hence, numerical models have been developed to investigate the in-flight particle melting behavior during spraying process by computational fluid dynamics program FLUENT V6.2©. The purpose of this study was focus on particles trajectory , heating history as well as its in-flight velocity under various carrier gas flow rate. In this study, the argon plasma jet is simulated. The alumina particle trajectories were tracked in Lagrangian manner. The results show that the spray-angle will increase as carrier gas flow rate enhance. The phenomenon is more obvious in internal powder injector than external one. As carrier gas flow rate increase , the maxium velocity of plasma jet will lift up and its maxium temperature will decrease. Particle with internal powder injection has better in-flight velocity and surface temperature than external one. This was due to the location of powder injector. Internal type was much closer to the heating and heart core of plasma jet which resulted in longer heating time and better melting status than external one. The former has higher particle mean surface temperature than latter as we further check particle counts distributions at standoff distance equal to nine centimeter. We use Biot number to analysis the melting state as particle surface temperature reach its melting point and integral mean thermal conductivity was used in this work. The results indicate that there still have fourteen percentage difference between surface temperature and its center point. As far as particle evaporation state be concered, particle residence time was shorter than its evaporation time. It seems like the phenomena wasn’t play an important role in this study. Furthermore, Particle injection velocity under different carrier gas flow rate resulted from ranges of corresponding particle size and injection location. Particle penetration depth mainly relied on its momentum and particle size. particle injection with external injector will be dispersed by plasma jet because of insufficient in momentum.The dispersion location for internal type was more concentrate than external type. At last but not least, CFD simulation tool have been developed in this study. It will help us analysis how the process variable affect the coating properties.